Electronic parking brake systems, also known as electronic, electrical and automatic parking brakes are increasingly replacing purely mechanical handbrakes in motor vehicles. The use of electronic parking brakes means that there is no need for the generally relatively large operating lever in the passenger space, with the result that the passenger space can be configured much more freely. Such a system is also much more user-friendly, as on the one hand the operator does not have to use a high level of force to secure or release the brake and on the other hand various functions, such as hill starts or releasing the brake when first starting after parking can be executed electronically and therefore also automatically. These advantageous features of an electronic parking brake system must however also be associated with a comparable or improved level of safety compared with the purely mechanical handbrake.
When controlling or in some instances regulating (here and throughout the document the term control must also be understood to include regulation) such a parking brake, by means of an electric motor transmission unit for example, there is generally a discrepancy between the position of the transmission and the force present at the brakes. This is due to the physical characteristics of the braking system and the force transfer device and is for example expressed in the form of hysteresis effects. In this context the term force transfer device should include the actuator and all parts which transfer forces to the brakes, as well as components on which the force of the actuator acts. As such unique assignment between the transfer position and brake force is difficult to achieve, a control operation is generally not just carried out by way of the transmission or engine position. A control operation can alternatively be carried out on the electronic parking brake by measuring the force at the force transfer device. Exclusive control or regulation by way of the force present at the force transfer device of the brake is however out of the question for safety reasons, as errors at the force transfer device and the above-mentioned hysteresis effects have to be taken into account. A combined force-travel controller is generally deployed with electronic parking brake systems for such reasons.
According to the prior art an “Apply parking brake-release parking brake” cycle takes place as follows: Starting from a start position, the parking brake is tensioned until a predefined force is achieved. This must be achieved within a predetermined travel with corresponding tolerances. During release the system then covers a permanently predetermined travel in the release direction based on the tension position. The position then achieved is determined as the new start position. This takes account of a changing travel-force characteristic due to wear of the brake device to a certain degree. If this characteristic changes further however, a residual force may be present in an undesirable manner after the parking brake has been released for example.
This can be remedied by opening the parking brake until no residual force is present, and then additionally covering a predetermined travel in the release direction. This method has the disadvantage that the brake has to cover unnecessarily long travel paths, as hysteresis effects generally influence the residual force values.